Field of the Invention
The invention relates to a voltage-controlled oscillator (VCO) for frequency modulation (FM).
Frequency modulation is one of the known modulation methods used in digital transmission systems. Such digital transmission systems are, for example, DECT (Digital Enhanced Cordless Telecommunication), which is used in cordless telephones, or BLUETOOTH, which has been configured as a standard for an interface for wireless communication over short distances.
In the frequency modulation method, the digitally coded information to be transmitted is modulated onto a carrier, for example a high-frequency sinusoidal oscillation. The standards described specify limit values for the modulation deviation. The modulation deviation or modulation excursion is here one half of the difference between the frequency with which a digital zero is coded and the frequency with which a digital one is coded if the information to be transmitted is binary coded. The modulation deviation must not exceed or drop below its limit values.
Highly integrated voltage-controlled oscillators which can be used in phase-locked loops, as needed in transmitters for such mobile radio systems, usually have tolerances since the frequency-determining integrated components are subject to manufacturing tolerances. VCOs, for example, can be constructed as LC oscillators in which the frequency of oscillation at a certain tuning voltage is influenced by tolerances of the components, especially inductance and capacitance. To achieve, in normal operation, a tuning range which is, for example, 100 MHz, it is usual to make the tuning range of a VCO greater when configuring its circuit, for example 600 MHz if the absolute frequency range is approx. 2.5 GHz. The tuning range is the range of an output signal of the VCO which is between a minimum tunable frequency and a maximum tunable frequency.
Such large tuning ranges, however, require that the capacitance in the resonant circuit, for example of LC oscillators having a fixed inductance, must be variable within a wide tuning range. However, this creates the problem that, for example if an adjustable modulation capacitance is used, it is not possible to achieve a constant modulation deviation over the entire tunable frequency range but, instead, the modulation deviation changes greatly over the tunable range and, in doing so, violates limit values of the specifications.
To illustrate the problems with a numerical example: a VCO with LC resonator has an inductance with a fixed inductance value of 2 nH. To achieve a tuning range of 2.2-2.8 GHz, the capacitance effective in the resonator must be adjustable from 1.62-2.62 pF. At the lower limit of the tuning range at 2.2 GHz, a modulation capacitance of 0.38 fF (femto-farad) is required for a modulation deviation of 160 KHz as required by BLUETOOTH. At the upper tuning limit of 2.8 GHz, however, the same modulation capacitance produces a modulation deviation of 328 KHz which is above the specified limit value.
It is already known to achieve a calibration of the fixed inductance value of the VCO in voltage-controlled oscillators by reducing a coil having too large an inductance value in a suitable manner in a calibration step during production. However, such calibration is both costly and time-consuming. Such calibration can be performed, for example, by using a laser. After the calibration of the inductance, the modulation deviation must be measured for each channel in which the VCO is to be operated. These measurement values can be stored in a baseband IC (integrated circuit) and conveyed to the radio frequency IC, for example via a databus, before each channel change in a transmit mode. This requires additional memory and furthermore the additional transmission of information before each channel change. In addition, a single measurement of the modulation deviation in the different channels is required. It is, therefore, desirable to be able to use a VCO without calibration and with constant modulation deviation, which requires only one single measurement of the modulation deviation.
Published, Non-Prosecuted German Patent Application No. DE 42 41 241 A1 discloses a voltage-controlled oscillator configured for frequency modulation. To achieve a constant frequency deviation, a coupling inductance is provided which couples the tuning input to a modulation capacitance. Such inductances, however, disadvantageously have a large space requirement.
It is accordingly an object of the invention to provide a voltage-controlled oscillator which overcomes the above-mentioned disadvantages of the heretofore-known configurations of this general type and which maintains a required modulation deviation over a large tuning range without elaborate calibration.
With the foregoing and other objects in view there is provided, in accordance with the invention, a voltage-controlled oscillator for frequency modulation, including:
an oscillator core including a tunable, frequency-determining resonant circuit component and a frequency-determining modulator component;
a first terminal for supplying a control voltage, the first terminal being coupled to the frequency-determining modulator component and being connected to the tunable, frequency-determining resonant circuit component for providing a frequency tuning;
a second terminal for supplying a modulation signal connected to the frequency-determining modulator component;
a third terminal for supplying a bias voltage;
an output terminal coupled to the oscillator core, the output terminal providing an output signal, the output signal being frequency modulated in dependence on the modulation signal and having a carrier frequency tuned in dependence on the control voltage; and
the frequency-determining modulator component including a first voltage-controlled capacitance and a second voltage-controlled capacitance connected in parallel to the first voltage-controlled capacitance, the first voltage-controlled capacitance being connected to the first terminal for supplying the control voltage and being controlled by the control voltage, the second voltage-controlled capacitance being connected the third terminal for supplying the bias voltage.
In other words, the object of the invention is achieved through the use of a voltage-controlled oscillator for frequency modulation which has an oscillator core having at least one tunable frequency-determining resonant circuit component which is connected to a terminal for supplying a control voltage, a frequency-determining modulator component which is connected to a terminal for supplying a modulation signal, the modulator component being coupled to the terminal for supplying the control voltage, and with an output terminal coupled to the oscillator core for picking up an output signal which is frequency modulated in dependence on the modulation signal and which has a carrier frequency tuned in dependence on the control voltage, the modulator component including a first voltage-controlled capacitance which, in order to be controlled, is connected to the terminal for supplying the control voltage, and the modulator component including a second voltage controlled capacitance which is connected in parallel with the first voltage-controlled capacitance and is coupled to a terminal for supplying a bias voltage.
The coupling of the modulator component to the terminal for supplying the control voltage can be constructed in a simple manner in such a way that the dependence of the modulation deviation on the frequency of the VCO set through the use of the resonant circuit component is compensated for.
Modulator component and resonator component can be jointly integrated in a resonator of the oscillator.
The oscillator core can have a resonator which can be constructed, for example, as LC oscillator or as RC oscillator.
The frequency-determining resonant circuit component can be, for example, the inductance or the capacitance of the resonator in the case of LC oscillators and the resistance or the capacitance of the oscillator in the case of RC oscillators.
In the case of LC oscillators, for example, the capacitance can be constructed as a varactor, usually as a capacitance diode with a junction capacitance which depends on the voltage applied.
Whereas a very great deviation of the oscillator frequency in dependence on deviations of an applied control voltage can be achieved through the use of the resonant circuit component, only relatively small frequency deviations, which are usually smaller by orders of magnitude than those caused by the resonant circuit component, are achieved through the use of the modulator component. Accordingly, the resonant circuit component can be tuned within a wide range whereas the modulator component can be tuned within a very small range. For example, frequency tuning from 2.2 to 2.8 GHz can be achieved through the use of the resonant circuit component whereas the modulation deviation which can be achieved through the use of the frequency-determining modulator component can be, for example, around 160 KHz. Coupling the terminal for supplying the control voltage to the modulator component makes it possible to keep the modulation deviation constant over the wide frequency range of the VCO. Accordingly, the modulation deviation is independent of the oscillating frequency of the resonator set.
The principle described can be implemented in digital or in analog circuit technology.
The modulator component can be connected directly to the terminal for supplying the control voltage.
According to the present invention, the modulator component includes a first voltage-controlled capacitance which, to be controlled, is connected to the terminal for supplying the control voltage. The voltage-controlled capacitance can be, for example, a varactor, particularly a capacitance diode with a junction capacitance which is constructed to be controllable.
Furthermore, the modulator component includes a second voltage-controlled capacitance which is connected in parallel with the first voltage-controlled capacitance and is coupled to a terminal for supplying a bias voltage.
Biasing, for example, a capacitance diode in the reverse direction prevents it from breaking down.
In a preferred embodiment of the present invention, a deattenuation amplifier or compensation amplifier is provided which is coupled to the oscillator core and exhibits the terminal at which the output signal of the oscillator is provided. Apart from the oscillator core already described, a voltage-controlled oscillator usually has a deattenuation or compensation amplifier which is constructed as active circuit component and provides a negative impedance for compensating for losses in the resonator. At the output of the deattenuation amplifier, an output signal can be picked up which has an oscillation or carrier frequency which is adjustable through the use of the control voltage. The deattenuation amplifier can include, for example, two cross-coupled transistors. The cross coupling can be, for example, capacitive.
Such an oscillator can be used, for example, in a phase-locked loop (PLL).
In another preferred embodiment of the present invention, the modulator component is connected to the terminal for supplying the modulation signal via at least one coupling resistor. Instead of the coupling resistor, other components for DC coupling can also be used.
In a further preferred embodiment of the invention, the resonant circuit component includes a third voltage-controlled capacitance which is connected via at least one further coupling resistor to a reference potential terminal and which, to be controlled, is connected to the terminal for supplying the control voltage.
In a further, advantageous embodiment of the present invention, the capacitance value of the third voltage-controlled capacitance or of the resonant circuit component, respectively, is greater by a factor of 10 or more than the capacitance value of the modulator component. The capacitance value of the modulator component can be composed of a number of varactors connected in parallel.
In a further advantageous embodiment of the present invention, first coupling capacitances are provided for coupling the resonant circuit component to at least one inductance and second coupling capacitances are provided for coupling the modulator component to the at least one inductance. In addition to the coupling described, the coupling capacitances can couple at the same time the oscillator core to the deattenuation amplifier and to a supply potential terminal.
In a further preferred embodiment of the invention, the capacitance value of the second coupling capacitance is smaller by a factor of 10 or more than that of the first coupling capacitances.
The fact that the resonant circuit component and its coupling capacitances have a capacitance value which is large compared with the capacitance value of the modulator component and its coupling capacitances has the result that a relatively large frequency deviation can be achieved through the use of the control voltage whereas only a relatively small frequency deviation corresponding to the modulation deviation can be achieved with the modulation signal.
In a further preferred embodiment of the present invention, the oscillator exhibits a balanced, i.e. symmetric, circuit configuration. The balanced circuit configuration, in particular, provides for high interference immunity.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a voltage-controlled oscillator for frequency modulation, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawing.